Wrapping update queries with joins in a subquery. Fixes #827 (#998)

spec/avram/queryable_spec.cr

@@ -1320,6 +1320,15 @@ describe Avram::Queryable do
       bucket = bucket.reload
       bucket.names.should eq(["Rey"])
     end
+
+    it "updates with joins" do
+      manager1 = ManagerFactory.create(&.name("Mr. Krabs"))
+      manager2 = ManagerFactory.create(&.name("Mr. Robot"))
+      EmployeeFactory.create(&.name("Spongebob Alderson").manager_id(manager1.id))
+      Employee::BaseQuery.new.where_manager(Manager::BaseQuery.new.id(manager1.id)).update(manager_id: manager2.id)
+      manager1.employees!.first?.should be_nil
+      manager2.employees!.first.name.should eq("Spongebob Alderson")
+    end
   end
 
   describe "#delete" do

src/avram/query_builder.cr

@@ -66,9 +66,17 @@ class Avram::QueryBuilder
     clone.statement_for_update!(params, returning?)
   end
 
+  # Creates the SQL for updating. If any joins are present, it
+  # moves the WHERE clause to a subquery allowing for joins
   def statement_for_update!(params, return_columns returning? : Bool = true)
     sql = ["UPDATE #{table}", set_sql_clause(params)]
-    sql += sql_condition_clauses
+    if joins_sql.presence
+      sql += ["WHERE EXISTS", "(", select_sql]
+      sql += sql_condition_clauses
+      sql += [")"]
+    else
+      sql += sql_condition_clauses
+    end
     sql += ["RETURNING #{@selections}"] if returning?
 
     join_sql sql